Measurement of ${}_Λ^{3}\mathrm{H}$ production in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV

The first measurement of $_{\Lambda}^{3}\mathrm{H}$ and $^3_ {\overline{\Lambda}}\overline{\mathrm{H}}$ differential production with respect to transverse momentum and centrality in Pb$-$Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV is presented. The $_{\Lambda}^{3}\mathrm{H}$ has been reconstructed via its two-charged-body decay channel, i.e., $_{\Lambda}^{3}\mathrm{H} \rightarrow {}^{3}\mathrm{He} + \pi^{-}$. A Blast-Wave model fit of the $p_{\rm T}$-differential spectra of all nuclear species measured by the ALICE collaboration suggests that the $_{\Lambda}^{3}\mathrm{H}$ kinetic freeze-out surface is consistent with that of other nuclei. The ratio between the integrated yields of $_{\Lambda}^{3}\mathrm{H}$ and $^3\mathrm{He}$ is compared to predictions from the statistical hadronisation model and the coalescence model, with the latter being favoured by the presented measurements.


Submitted to: PLB
e-Print: arXiv:2405.19839 | PDF | inSPIRE
Figure group

Figure 1

Invariant-mass distribution of selected ${}_{\Lambda}^{3}\mathrm{H}$ candidates in the centrality class 0$-$10% and $p_{\rm T}$ interval 3 $<$ $p_{\rm T}$ $<$ 3.5 $\mathrm{Ge\kern-.1emV/}c$ fitted with a function which is the sum of a double-sided Crystal Ball signal and an exponential background. Vertical lines represent the statistical uncertainties.

Figure 2

Deuteron (d), antitriton ($\mathrm{\overline{t}}$), ${}^3\mathrm{He}$ , and ${}_{\Lambda}^{3}\mathrm{H}$ spectra measured in Pb$-$Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV. Each panel shows a centrality interval and different nuclei are reported with different colours. For ${}_{\Lambda}^{3}\mathrm{H}$, the average spectra between particles and antiparticles is employed. The boxes represent the systematic uncertainties, while the vertical lines are the statistical ones. The combined Blast-Wave fit parameters for d, $\mathrm{\overline{t}}$, ${}^3\mathrm{He}$, and ${}_{\Lambda}^{3}\mathrm{H}$ are listed in Table 1.

Figure 3

Yield ratio of ${}_{\Lambda}^{3}\mathrm{H}$ to $^3\mathrm{He}$ together with theoretical predictions as a function of multiplicity. In the left panel, the results of this analysis are compared with the ALICE measurement at $\sqrt{s_{\mathrm{NN}}}$ = 2.76 TeV  . Vertical lines are used for the statistical uncertainties and boxes for the systematic ones. For each centrality interval the $\langle \mathrm{d}N_\mathrm{ch}/\mathrm{d}\eta\rangle$ is taken from Ref. [68] and the ${}^{3}\mathrm{He}$ yield from Ref. [48]. The dense orange dashed line represents the expectation of SHM, while the other three sets of lines stand for coalescence model with different $B_\Lambda$ hypotheses. The coalescence prediction with world average $B_\Lambda$ is displayed with a 1$\sigma$ uncertainty as the filled area, both lines and shadowed areas are linear interpolations of the available model calculations . In the right panel, the results of recent STAR measurement are shown for comparison .

Figure 4

The ${}_{\Lambda}^{3}\mathrm{H}/{}^3\mathrm{He}$ yield ratio together with theoretical predictions in different centrality intervals as a function of $p_{\rm T}$. The vertical lines and boxes represent the statistical and systematic uncertainties, respectively. The pink solid lines are calculated as the ratios of the Blast-Wave fit functions for ${}^3\mathrm{He}$ and ${}_{\Lambda}^{3}\mathrm{H}$. The other three curves stand for the predictions of the coalescence model with different $B_\Lambda$ hypotheses. The coalescence prediction with world average $B_\Lambda$ is displayed with a 1$\sigma$ uncertainty band .